1. Field of the Invention
The invention relates to an antenna device for transceiving radio signals.
2. Description of the Related Art
Referring to FIGS. 1 and 2, a conventional antenna device 1 is shown to include: a connector 11; an upright antenna rod 12 connected electrically to and extending upward from the connector 11; an outer sleeve body 14 mounted on the connector 11 and permitting extension of the antenna rod 12 therethrough; a copper tube 15 mounted on and surrounding the antenna rod 12, and having a closed upper end 151 that contacts electrically a tip of the antenna rod 12, and an open lower end 152 extending into the outer sleeve body 14; and a tubular outer cap 16 mounted on the outer sleeve body 14 for covering the copper tube 15.
The conventional antenna device 1 is capable of transceiving electromagnetic waves, the frequencies of which are determined based on a resonance frequency created by the antenna rod 12 and the copper tube 15. In other words, the resonance frequency is associated with an inner diameter and a length of the copper tube 15. Therefore, if the conventional antenna device 1 is designed to have a different frequency band, the size of the copper tube 15 should be changed. However, since the copper tube 15 is fabricated through a lathe procedure, it is difficult to exactly control the size of the cooper tube 15. Moreover, since the copper tube 15 has a certain thickness, the conventional antenna device 1 has a relatively high cost.